In recent years, there has been a growing interest in UWB (Ultra-Wide-Band) as wireless technology in the ultra-wideband of several GHz. The UWB wireless technology refers to a communication system that uses, instead of a carrier wave employed in general radio technology, very short pulses of nsec (ten to nine sec) or less.
Regarding the UWB wireless technology using short pulses, the bandwidth corresponds to the wideband of several GHz and thus the UWB wireless technology is called wideband wireless technology. Further, because of the use of the very short pulses, the UWB wireless technology occupies a considerably wide bandwidth, while the frequency spectrum power density is very low. Therefore, as compared with other types of wireless radio technology, the UWB wireless technology is superior in privacy and concealment. In addition, because of no carrier wave, the transmission and reception apparatuses do not require modulation and demodulation circuits, keeping power consumption low. Thus, the UWB wireless technology is superior in some respects as compared with conventional wireless radio technology. Depending on trends in legal regulations in Japan, development of products using the UWB wireless technology is of urgent necessity.
The IEEE (Institute of Electrical and Electronic Engineers) in the US that is the world's greatest standards-setting organization is now studying and establishing proposed standards for use of the UWB wireless technology. Standards have been established under IEEE 802.15.3a for a physical (PHY) layer and IEEE 802.15.3 for a MAC (Medium Access Control) layer.
A description is given in connection with FIG. 17, concerning IEEE 802.15.3 defined as proposed standards for the UWB wireless technology.
As shown in FIG. 17, a network protocol stack is defined by IEEE 802.15.3 with a PHY layer 1001, a MAC layer 1002 and a FCS (Frame Check Sequence) layer 1003 as well as a DEV Device Management Entity (hereinafter referred to as DME) 1004 controlling each layer. PHY layer 1001 and MAC layer 1002 have such control units as a MAC layer management entity (hereinafter referred to as MLME) and a PHY layer management entity (hereinafter referred to as PLME) 1006 for control and management of data transmission/reception in each layer.
More specifically, PHY layer 1001 defines establishment of physical links as well as electrical and mechanical standards and determines an actual data transmission rate and radio frequency band at physical levels. MAC layer 1002 actually determines a communication path between adjacent nodes and transmits/receives data through PHY layer 1001. Basic data transmission/reception can be implemented by these two layers.
FCS layer 1003 is a layer converting data for each application for MAC layer 1002. In general, any portions higher than MAC layer 1002 correspond to applications including FCS layer 1003 and DME 1004. DME 1004 manages control information 1011 to manage and control operations between the layers.
As a method of access between layers, a method of access is defined that is done through an interface point between the layers called SAP (service access point) and that is appropriate for the layers. Between the layers, data and control information are communicated in a manner conforming to the access method.
More specifically, data 1007 sent from an application is transmitted through a FCSL-SAP 1008 that is an interface point between FCS layer 1003 and its higher layer, a MAC-SAP 1009 that is an interface point between FCS layer 1003 and MAC layer 1002 and a PHY-SAP 1010 that is an interface point between MAC layer 1002 and PHY layer 1001.
Further, control information 1011 is communicated between DME 1004 and MLME 1005 of MAC layer 1002 through a MLME-SAP 1012 and between DME 1004 and PLME 1006 of PHY layer 1001 through a PLME-SAP 1013. Control information 1011 is also communicated through MLME 1005 of MAC layer 1002 between DME 1004 and PLME 1006 of PHY layer 1001 through a MLME-PLME-SAP 1014.
IEEE 802.15.3 employs TDMA (Time Division Multiple Access) as a communication system. Over the network as shown in FIG. 18 that is called Piconet established using the UWB wireless technology, data is transmitted/received in accordance with TDMA between a Piconet Coordinator (hereinafter referred to as PNC) that performs centralized management of the network and other devices (hereinafter referred to as DEV). For convenience of the description, FIG. 18 shows piconet 404 including PNC 401 and one device DEV 402.
For communication to be achieved between PNC 401 and DEV 402, PNC 401 sends a beacon frame 403 to each DEV (DEV 402 in the case shown in FIG. 18) belonging to piconet 404 managed by PNC 401. DEV 402 belonging to piconet 404 receives beacon frame 403 from PNC 401 to share time with PNC 401 and accordingly time sharing within piconet 404 is implemented.
The beacon frame is described in connection with FIG. 19. As shown in FIG. 19, PNC 401 that manages piconet 404 conceptually divides a transmission path at constant time intervals into periods T (sec), and successively transmits, at each period T, beacon frame 405 to each DEV (DEV 402 in the case shown in FIG. 18) belonging to piconet 404. The sections at the constant time intervals are each called superframe. A (n−1)-th beacon frame B(n−1) and the n-th beacon frame B(n) are separated to generate a time slot and the time slot corresponding to a predetermined time is used to transmit data between PNC 401 and DEV 402.
The superframe is generated in the MAC layer of the PNC. From an application layer of the PNC, data to be transmitted to the DEV is provided through the MAC-SAP and the MLME-SAP to the MAC layer. In the MAC layer of the PNC, the provided data is structured in the superframe according to the method as defined by the standards of IEEE 802.15.3 and transmitted through the PHY-SAP to the PHY layer. Thus, in order for data to be transmitted from the PNC to the DEV, data and control information are communicated through the MAC-SAP and the NLME-SAP that are interface points between the MAC layer and the application layer.
The UWB wireless technology finds an application, utilizing the high-speed and wideband characteristics, in achieving real-time transmission of AV (Audio Video) data of the HDTV (High Definition Television) class.
Currently, BS digital broadcasting and terrestrial digital broadcasting are standardized by the ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-1. Video using a system according to MPEG (Moving Pictures Experts Group)-2 with a transmission rate from several Mbps (106 bits/second) to several tens of Mbps as well as high-efficiency coding information data of audio are arranged in packets that are sent by multiplexing transmission together with synchronization information and thereby implement digital broadcasting.
FIG. 20 shows a method of establishing synchronization of MPEG2-TS (Transport Stream) that is real-time transmission technology.
With reference to FIG. 20, according to the standards of MPEG2-TS, layers lower than the application layer of the transmission/reception apparatus are synchronized with each other. In order to synchronize the time of respective application layers that include a MPEG2 encoding apparatus 1031 and a MPEG2 decoding apparatus 1032, a program clock reference (hereinafter referred to as PCR) 1033 is used that is information indicating time. Specifically, MPEG2 encoding apparatus 1031 includes a system time clock (hereinafter referred to as STC) 1037 that is a clock of the encoding apparatus and generates PCR 1033 from the transmission time TA of a MPEG2-TS packet 1034 that is obtained from STC 1037. Once in at least 100 msec (10−3), PCR 1033 is embedded in MPEG2-TS packet 1034 to be transmitted from MPEG2 encoding apparatus 1031 to MPEG2 decoding apparatus 1032. The PCR that is time information embedded in the MPEG2-TS packet is also called time stamp.
MPEG2 decoding apparatus 1032 also includes a STC 1035 that is a clock of the decoding apparatus. MPEG2 decoding apparatus 1032 compares, with PCR 1033, time TB at which it receives MPEG2-TS packet 1034 in which PCR 1033 obtained from STC 1035 is embedded, controls a phase-locked circuit (Phase-Locked Loop, hereinafter referred to as PLL) 1036 included in MPEG2 decoding apparatus 1032, and matches time TB at which MPEG2-TS packet 1034 is received with time TA at which MPEG2-TS packet 1034 is transmitted.
According to the standards of MPEG2-TS, respective application layers including the encoding apparatus and the decoding apparatus are synchronized in time with each other by the above-described method. Then, for stabilizing the operation of PLL 1036 included in MPEG2 decoding apparatus 1032, it is necessary that MPEG2-TS packets are transmitted to MPEG2 decoding apparatus 1032 so that there is no or minimum occurrence of jitter that is called variation in transmission, namely that time intervals at which the packets are output from MPEG2 encoding apparatus 1031 are kept. If the jitter is large, the time information cannot be matched between the encoding apparatus and the decoding apparatus. As a result, images could distort and, in some cases, the images could not be reproduced. According to the standards of MPEG-2, the tolerable error of the PCR is defined as ±500 nsec.
A description is given in connection with FIG. 21 regarding occurrence of jitter that is variation in transmission.
Referring to FIG. 21, when MPEG2-TS packets (#1, #2, #3 . . . ) are transmitted on a transmission line from the transmitter to the receiver, a certain transmission delay occurs. In an ideal transmission system, when a certain transmission delay time has passed since a MPEG-2 TS packet is transmitted, the packet is received by the receiver and, the time intervals at which the packets are received are kept at time intervals T1, T2, T3 . . . at which the packets are transmitted by the transmitter. In actual transmission, however, except for the case where a transmission path for communication is poor, depending on a difference in operating frequency between the transmitter and the receiver as well as occurrence of processing task, for example, there is generated a difference between an ideal receiving time and an actual receiving time, namely variations 1051 in the intervals at which the packets are received. The variations are called jitter.
Patent Document 1: Japanese Patent Laying-Open No. 2000-78123